1. Technical Field
The present disclosure relates to a phosphor and a light emitting device using the same.
2. Background Art
There have been developed light emitting devices that can emit light of various hues, by using combinations of a light source and a phosphor which can be excited by light from the light source to emit light of different hue than that of the light source, based on the principle of light-color mixing. For example, primary light in a short wavelength region corresponding to ultraviolet light to visible light is emitted from a light emitting element and with the emitted light, a phosphor is excited. As a result, at least a portion of primary light is wavelength-converted, and red, blue, green, or the like, a light of desired color can be obtained. Further, a mixed color light of white light can be emitted by mixing those colors of light.
Light emitting devices employing a light emitting diode (hereinafter may be referred to as an “LED”) are used in a number of areas such as a signal light, a mobile phone, various illumination, an in-vehicle display device, and various display devices. Particularly, a light emitting device constituted with a combination of an LED and a phosphor has been actively applied to a backlight for a liquid crystal display, a small-type stroboscope, or the like. Recently, applications to lighting devices have been developed, and with utilizing advantageous properties such as long operational life and free of mercury, and such lighting devices are expected as environmentally-conscious light sources which can be replaced with fluorescent lamps.
Examples of the structure of the light emitting device include a combination of a blue LED and a yellow phosphor (for example, see JP 3503139 B). The light emitting device is configured to emit a mixed color light of white light, which is obtained by mixing blue light from the LED and yellow light which is a wavelength-converted light of a portion of blue light from the LED converted by a yellow phosphor into yellow light. For this purpose, the phosphor used in the light emitting device is required to have properties that allow for being efficiently excited by blue light of wavelength of 420 nm to 470 nm emitted from the LED and emitting yellow light.
Examples of such yellow phosphor include cerium-activated yttrium aluminum garnet-based phosphors. The examples also include phosphors obtained from such yellow phosphors by substituting portion of Y with Lu, Tb, Gd, or the like, or portion of Al with Ga or the like. The cerium-activated yttrium aluminum garnet-based phosphors can be represented by a general formula (Y, Lu, Tb, Gd)3(Al, Ga)5O12:Ce, which allows for wide wavelength adjustment by adjusting the composition.
In the case of employing a typical light emitting device in which the yellow phosphor and a blue LED are used in combination, for the backlight of liquid crystal displays or for lighting devices, green component (480 to 530 nm) and red component (600 nm or greater) may be insufficient, so that an improvement in the color reproduction range and/or an improvement in the color rendering properties are required. The color reproduction range of a liquid crystal display device and the color rendering properties of a lighting device can be improved by combining a phosphor to emit light of short wavelength in a blue-green color, a green color, or a yellow-green color, and a phosphor to emit light of long wavelength in an orange color or a red color, in place of the yellow phosphor, or in addition to the yellow phosphor.
Examples of known such phosphors include a silicate phosphor, a phosphate phosphor, an aluminate phosphor, a borate phosphor, a sulfide phosphor, and an oxysulfide phosphor. Further, there have been proposed as an alternative to these phosphors, that are phosphors with less deterioration in the luminance even under high energy excitation, such as a sialon phosphor, an oxynitride phosphor, and a nitride phosphor, whose host crystals are an inorganic crystal which contains nitrogen in its crystal structure.
Of those phosphors, as an example of nitride phosphors, a red phosphor which has CaAlSiN3 as a host crystal activated with Eu2+ (hereinafter may be referred to as a “CASN phosphor”) has been known (for example, see JP 3837588B). The use of a CASN phosphor has an effect for improving the color rendering properties of the light emitting device. Further, there has been known a phosphor (Sr, Ca) AlSiN3:Eu (hereinafter may be called as a “SCASN phosphor”) in which a part of Ca in CaAlSiN3:Eu is substituted with Sr to increase the luminous flux of the light emitting device, in which, the more the content of Sr, the shorter the wavelength will be. (For example, see JP 2006-8721A.)
The emission peak wavelength of the CASN phosphors may be about 650 nm and the SCASN phosphors exhibit an emission at 610 nm to 650 nm which is in a shorter wavelength than the CASN phosphors. With the use of a red phosphor to emit light of a short wavelength, a light emitting device of higher brightness due to visibility can be obtained, while increasing the red light component. Thus, the SCASN phosphor is very promising red phosphor.
The SCASN phosphor can be manufactured through the operations summarized as below. The powder of raw materials of calcium nitride (Ca3N2), strontium nitride (Sr3N2), silicon nitride (Si3N4), aluminum nitride (AlN), and europium nitride (EuN) are mixed at a rate of Ca:Sr:Al:Si:Eu=0.1984:0.7936:1:1:0.008 in a glove box in a nitrogen atmosphere. The mixture is sieved through a 500 μm sieve to naturally fall into and fill a boron nitride crucible. Then the crucible is placed in a graphite resistance heating-type electric furnace and is subjected to sintering by using a gas-pressure sintering method at a temperature of 1800° C. for 2 hours in nitrogen gas of 1 MPa, thus a SCASN phosphor is manufactured.
However this synthesis method was found to produce a phosphor with low characteristics that exhibits the emission intensity of about 80% with respect to a CASN phosphor. This was caused by that with this synthesis condition, a stable CASN phosphor can be obtained but a SCASN phosphor cannot be stably exist and is gradually decomposed into different compounds (Sr2SiN5, AlN, or the like), so that a substantially pure SCASN phosphor was not able to be produced. Accordingly, methods for improving the characteristics have been studied.
For this purpose, a method have been proposed in which, without using calcium nitride or silicon nitride or aluminum nitride, metals such as calcium, strontium, silicon, aluminum, and europium are alloyed, and the powder obtained by pulverizing the alloy is nitride.
For example, see JP 2006-307182A.
Also see H. Watanabe, et al. “Synthetic Method and Luminescence Properties of SrxCa1-xAlSiN3:Eu2+ Mixed Nitride Phosphors” Journal of The Electrochemical Society, 155 (3) F31-F36 (2008).
However, any phosphors and the method of manufacturing those phosphors described above regard merely a synthesis and emission luminance of a SCASN phosphor, and the control of the shape of the emission spectrum by finely controlling the chemical composition has not been examined. Also, the SCASN phosphors are used to increase the red light component of the light emitting device to improve the color reproduction range and color rendering properties. Along with a requirement for further improvement in the luminous flux of the light emitting device, a higher luminance is also required to the SCASN phosphors.
The present invention is devised to solve the problems described above. That is, one object of the present disclosure is to provide phosphors in which the visibility component is increased to improve luminance of the SCASN phosphors.